Key, Value vs Item, Index

MUSTDOS · April 7, 2024, 11:29pm

PacktPublishing/Crystal-Programming/blob/main/Chapter10/macros2.cr

macro def_method(name)
  def {{name.id}}
    puts "Hi"
  end
end

def_method foo

foo

github.com

PacktPublishing/Crystal-Programming/blob/main/Chapter10/macros3.cr

macro def_methods(*numbers, only_odd = false)
  {% for num, idx in numbers %}
    {% if !only_odd || (num % 2) != 0 %}
      # Returns the number at index {{idx}}.
      def {{"number_#{idx}".id}}
        {{num}}
      end
    {% end %}
  {% end %}
  {{debug}}
end

def_methods 1, 3, 6, only_odd: true

pp number_0
pp number_1

I’ve been looking at both codes and noticed that splat reverses the intuitive “index then item”
why is it this way?

Blacksmoke16 · April 7, 2024, 11:33pm

Sorry I’m not sure I follow what you mean by:

splat reverses the intuitive “index then item”

MUSTDOS · April 7, 2024, 11:55pm

I find it odd that splat forces me to “for value, index in collection” while tuple/hash needs “for key, value in tuple/hash”

MUSTDOS · April 7, 2024, 11:56pm

github.com

PacktPublishing/Crystal-Programming/blob/main/Chapter10/macros4.cr

{% begin %}
  {% hash = {"foo" => "bar", "biz" => "baz"} %}

  {% for key, value in hash %}
    puts "#{{{key}}}=#{{{value}}}"
  {% end %}
{% end %}

{% begin %}
  {% arr = [1, 2, 3] %}
  {% hash = {} of Nil => Nil %}

  {% arr.each { |v| hash[v] = v * 2 } %}

  puts({{hash}})
{% end %}

Ooops!, here’s the correct link for to compare macros 3 vs 4 with

Blacksmoke16 · April 8, 2024, 12:15am

I’m still not sure what you mean by splat in this context, but I think I see what you’re talking about now. I’m pretty sure the gist of it is the macro for loop arguments are setup the same as the non-macro context. E.g. with a normal array and #each_with_index it’s value, index. Whereas with a normal hash/named tuple and #each it’s key, value.

MUSTDOS · April 8, 2024, 12:42am

Thanks, all better.

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Key, Value vs Item, Index

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